Field of the Invention
The present invention relates to a position measurement system, a calibration method for the position measurement system, a lithographic apparatus comprising such a position measurement system and a device manufacturing method in which use is made of such a position measurement system.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Lithographic apparatus usually comprise a plurality of moveable components of which the position needs to be known very accurately to obtain an accurate transfer of the pattern to the target portion. Key in obtaining the position are position measurement systems. Depending on the application and the required accuracy, the position measurement systems may be based on the principle of letting two measurement waves interfere with each other after at least one of the measurement waves has interacted with an object, e.g. a target, on the moveable component to change the phase of the measurement wave(s). By detecting the intensity after interference, a signal can be obtained that represents the phase difference between the measurement waves. The signal is thus also representative of the position of the moveable component.
However, position measurement systems need to be calibrated e.g. in order to avoid measurement errors that have a periodic dependence on the object position. The calibration needs to be repeated regularly, since the effects to be compensated tend to vary with time and operating point. Currently, the position measurement systems require the moveable component to move over a substantial distance during calibration, resulting in a significant interruption of normal operation of the component.